The present invention relates to a method for drying and/or cooling a paper web or equivalent in which the web is passed over the circumference of a revolving roll or equivalent on support of a face of a support wire or equivalent that faces the roll. The web is dried and/or cooled by means of a gas.
Further, the present invention relates to a device for drying and/or cooling of a paper web or equivalent, which comprises a revolving roll or equivalent provided with openings placed in the mantle thereof. In an interior of the roll, a system of gas ducts is formed, the web to be dried being arranged to run over the circumference of the roll preferably on support of a support wire or equivalent.
The highest web speeds in paper machines are currently of an order of about 25 meters per second, but before long, the speed range of from about 25 m/s to about 40 m/s is also likely to be realized. Even with the highest speeds that are employed now, and with the ever higher speeds in the future, the dryer section of a paper machine has especially become and will be a bottle-neck for the runnability of a paper machine.
In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In the twin-wire draw, the groups of drying cylinders comprise two wires, which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible of fluttering, possibly resulting in web breaks. In the single-wire draw, each group of drying cylinders comprises only one drying wire on whose support the web runs through the whole group so that, on the drying cylinders, the drying wire presses the web against the heated cylinder faces, and on the reversing cylinders between the drying cylinders, the web remains at the side of the outside curve, i.e., the drying wire is between the web and the reversing cylinders. Thus, in single-wire draw, the drying cylinders are situated outside the wire loop, and the reversing cylinders are situated inside the loop. In the prior art normal groups with single-wire draw, the heated drying cylinders are placed in the upper row, and the reversing cylinders are placed in the lower row, the rows being generally substantially horizontal and parallel to one another. So-called inverted groups with single-wire draw are also known, in which the heated drying cylinders are placed in the lower row and the reversing suction cylinders or rolls in the upper row, the substantial objective being to dry the web from the side opposite in relation to a normal group with single-wire draw.
In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly the groups with single-wire draw similar to those mentioned above.
In the area of the dryer section of a paper machine, various problems have occurred, for which the present invention suggests novel efficient solutions.
These problems include the large length of the dryer section, which increases the costs of the dryer section and of the machine hall. Problems have also been caused by the difference in speed between the paper web and the wires, which has resulted in wear of the wires and, at the worst, even to paper breaks in the dryer section. Problems may also have occurred in the controllability of the web draw and in the runnability of the web.
With respect to the prior art, reference is made to Finnish Patent Application No. 2919/71 which describes a continuous dryer for paper products in which two cylinders with adjustable temperatures are used. The cylinders are placed at a short distance from one another on the joint portion of the runs of carrier belts penetrable by air. The belts are guided towards the starting point of their joint run by means of cylinders which blow hot air through their side walls and which cylinders have been called air-blow cylinders. In this prior art construction, heat is always transferred to the paper by the intermediate of a wire, in which case the wire that is employed must be capable of storing of heat. Further, the wire is penetrable by air, because the preheating takes place by the principle of blowing through the wire. It follows from this description that it is always necessary to use a woven metal wire. Also in this construction, the outlet gas is passed through the wire to outside the cylinder, and the paper is always carried between two wires. The construction of the blow cylinder in itself has not been described, but it has just been stated that, in the cylinders, an inside partition wall is employed.
With respect to the prior art, reference is also made to Finnish Laid-Open Publication No. 62,573, in which a dryer for drying a continuous paper-fibre sheet is described. In this dryer, there is a rotatable drum and means for feeding dry gas into the interior of this rotatable drum and means for making the gas to flow out of the rotatable drum. The material to be dried is not supported by a wire or equivalent and runs around the drum on support of strips. The drying gas spreads onto the face of the material to be dried along particular passages placed on the face of the drum, in which passages the gas can spread in the direction of the circumference only, because the strips for supporting the material to be dried separate the passages from one another. The trapeze-like strips parallel to the circumference intensify the flow of the drying gas in the direction parallel to the circumference. The gas distribution equipment is stationary, i.e., any leakage from the drying gas to the outlet gas take place inside the drum. This results in a certain percentage of loss of gas.